The present invention relates to an exhaust emission control system of an engine, and particularly to an exhaust emission control system which is provided on an exhaust passage with a NOx catalyst which purifies NOx in exhaust gas.
Conventionally, exhaust emission control systems of engines, which include a SCR catalyst disposed in an exhaust passage of the engine and for purifying NOx within exhaust gas by causing a reaction with NH3 and a NOx storage catalyst that stores (occludes) NOx contained in exhaust gas when an air-fuel ratio of the exhaust gas is lean (i.e., λ>1, larger than a theoretical air-fuel ratio) are known (e.g., see JP3518398B). Such a NOx storage catalyst further reduces the stored NOx when the air-fuel ratio is approximately equal to stoichiometric (i.e., λ≈1, approximately equal to the theoretical air-fuel ratio) or is rich (i.e., λ<1, smaller than the theoretical air-fuel ratio). In the exhaust emission control system of the engine, the SCR catalyst purifies NOx when an engine speed and an engine load are high, i.e., the temperature of the SCR catalyst is high, and otherwise the NOx catalyst purifies NOx.
Further, JP2010-112345A discloses an art for performing NOx purification with a SCR catalyst by adsorbing NH3 generated in a NOx reduction control of a NOx catalyst, instead of providing a urea injector for injecting urea to the SCR catalyst.
With the art of JP3518398B, if a urea injection by a urea injector is not performed normally within an operating range of the engine where the SCR catalyst purifies NOx, i.e., the NOx catalyst does not purify NOx, the NOx purification by the SCR catalyst becomes insufficient, and a problem arises in that a large amount of NOx is discharged.
Therefore, NH3 generated in the NOx reduction control of the NOx catalyst may be supplied to the SCR catalyst (see JP2010-112345A).
However, the amount of NH3 generated in the NOx reduction control by the NOx catalyst is comparatively small, thus causing not enough NH3 to be supplied to the SCR catalyst to sufficiently purify NOx.